The present invention relates to a method and apparatus for pressure testing production tubing in a producing well having a downhole progressing cavity pump.
It is common practice to use a downhole pump to provide artificial lift to bring oil to the surface of a producing well after reservoir pressure has declined to the point where the well will no longer produce by natural energy. One form of downhole pump commonly used is the progressing cavity pump (PCP). The PCP is considered to be a positive displacement pump which is actuated by rotary motion. It consists of a single helical rotor rolling eccentrically in a double threaded helical stator of twice the pitch length. When actuated, the PCP produces fluid in a nonpulsating, continuous flow fashion and is approximately twice as efficient as a reciprocating rod pump.
A conventional oilwell installation incorporates the stator of a PCP to the production tubing string. The rotor is driven by a sucker rod string which is connected at its lower end to the rotor and extends inside the production tubing up to the surface. The sucker rod string is driven in rotary fashion by a surface drive head actuating the PCP.
Because the rotor of a PCP rolls in an eccentric motion inside the stator, this eccentric motion is imparted to the sucker rod string causing it to contact the inside walls of the production tubing, often producing a leak. For this reason, oilwell operators routinely pressure test the production tubing of wells fitted with a PCP for leaks.
The typical method for pressure testing production tubing requires the sucker rod string with rotor to be pulled out of the oilwell. A pressure actuated dart is pumped down the tubing until it seats inside a seating nipple formed on the inside walls of the production tubing above the PCP stator. With the dart in sealing engagement with the seating nipple, fluid is then pumped into the tubing and the pressure is allowed to build up. Loss of pressure indicates a leak and the tubing string must be pulled and repaired. If there are no leaks, a fishing tool is run into the well on a wireline, the dart is withdrawn, the sucker rod string and rotor is run back in the well and pumping is recommenced.
Production tube pressure testing is costly. It requires a service rig to be brought to the well head and a derrick erected to withdraw the sucker rod string. Not only are the rig costs considerable, but because the overall down-time of the well during testing is about 6-8 hours, a substantial loss of revenue is involved. In addition, the requirement to pull out and run in the sucker rod string each time testing is carried out causes wear on the sucker rod couplings.